First we determined that the conditioned media of MDA-MB-231, a breast cancer cell line expressing IL-11[13,20], gave rise to a population of cells which can form osteoclasts in response to RANKL and M-CSF treatment (Figure 1), indicating that IL-11 may play an important role in osteoclastogenesis by regulating the development and/or survival of osteoclast progenitor cells. This evidence concerns the gene CSF1 and breast cancer.